1. Field of the Invention
This invention relates to encapsulated electronic circuits and, in particular, to shock-resistant electronic circuit assemblies that find use in blasting components.
2. Related Art
Electronic timing circuits for firing detonators after a predetermined, electronically-controlled delay period are known. The delay period is measured from the receipt of an initiation signal that may provide power for the timing circuit. Thus, U.S. Pat. No. 5,133,257 to Jonsson, issued Jul. 28, 1992, discloses an ignition system comprising a piezoelectric transducer that can be disposed next to a detonating cord branch line. When the detonating cord detonates, a shock wave is imposed on the piezoelectric transducer which then produces an electrical pulse. The electrical energy from the transducer is stored in a capacitor which provides power for a timer. After a predetermined delay, the timer allows the remaining stored energy in the capacitor to fire an ignition head in the detonator. The ignition head initiates explosive material, thus providing the explosive output for the detonator. Electronic delay circuits may also be used to initiate bridge elements such as a semiconductor bridge as described, e.g., in U.S. Pat. No. 4,708,060 to Bickes, Jr. et al, issued Nov. 24, 1987, or a tungsten bridge as described, e.g., in U.S. Pat. No. 4,976,200 to Benson et al, issued Dec. 11, 1990. Other electronic delay circuits are seen in U.S. Pat. No. 5,173,569 to Pallanck et al, issued Dec. 22, 1992; in U.S. Pat. No. 5,377,592 to Rode et al, issued Jan. 3, 1995; and in U.S. Pat. No. 5,435,248 to Rode et al, issued Jul. 25, 1995. These patents generally suggest that the electronic circuitry be molded within a suitable plastic package, casing or "potting compound". See, e.g., the Jonsson Patent at column 2, lines 42-50; the Pallanck et al Patent at column 3, lines 32-35; the Rode et al Patent (U.S. Pat No. 5,377,592) at column 9, lines 30-33 and the Rode et al Patent (U.S. Pat. No. 5,435,248) at column 7, lines 9-13. The stated purpose for such enclosures is to protect the electronic components and minimize the chances of detonation or damage by mechanical impact. No particular configuration or material for such casings is disclosed in these patents.
U.S. Pat. No. 4,869,170 to Dahmberg et al, issued Sep. 26, 1989, discloses a detonator comprising a circuit encased in an encapsulation (70) within an elongate enclosure. The encapsulation extends along only a portion of the length of the enclosure, but where the encapsulation is co-extensive with the enclosure, it appears to make full contact with the longitudinal interior surface of the enclosure, i.e., there are no gaps between the encapsulation and the enclosure.
U.S. Pat. No. 4,118,861 to Palmisano, issued Oct. 10, 1978, discloses the use of a plurality of loose pellets used as packing material for a circuit board in an enclosure.
U.S. Pat. No. 4,712,477 to Aikou et al, issued Dec. 15, 1987, discloses an electronic delay detonator in which a delay circuit is suspended in a plastic casing by a plastic plug. The use of a plastic casing and the absence of an encapsulation about the delay circuit indicates that the disclosed detonator is vulnerable to lesser external forces than could be withstood by a conventional detonator having a metal housing.
U.S. Pat. No. 5,589,657 to Gessel et al, issued Dec. 31, 1996, discloses a detonating apparatus comprising a conventional electronic delay detonator (1) placed in a glass Dewar vessel (3). The Dewar vessel is mounted within a sheath (4) by elastomeric plugs (5, 10) at either end. The plugs extend beyond the ends of the Dewar vessel. No part of either plug is disposed between the detonator and the longitudinal, cylindrical internal surface of the sheath.
U.S. Pat. No. 4,656,442 to Hayakawa, issued Apr. 7, 1987, discloses an integrated circuit package comprising various discrete circuit elements encased in an encapsulation with only input and output leads protruding from the encapsulation.
U.S. Pat. No. 4,400,858 to Goiffon et al, issued Aug. 30, 1983, discloses a downhole electric plug comprising a circuit board suspended in a tube that constitutes the enclosure for the circuit. The circuit on the circuit board emits signals as part of a radio telemetry system. The circuit board is suspended by metallic spring clips mounted periodically along the board. The circuit generates heat during its operation and the spring clips serve as heat sinks. There is no casing about the circuit board between the circuit board and the enclosure, none of the clips encases the circuit and each clip makes substantially full circumferential contact in the regions where the clip is axially co-extensive with the surrounding tube.
The substance of U.S. patent application Ser. No. 08/742,296, filed Nov. 1, 1996, which is a parent of this continuation-in-part application, is contained in South African patent application No. 97/9757, which was laid open for inspection May 1, 1998.